This invention relates to image forming apparatus having separate devices for applying toner to and for cleaning toner from an image surface. Although not limited thereto, it is particularly usable in image forming apparatus in which a uniform layer of toner is applied to a surface, the toner is imagewise tacked to the surface, and untacked toner is cleaned off the surface. It can also be used in conventional electrostatic image forming apparatus.
U.S. patent application Ser. Nos. 07/632,698, now U.S. Pat. No. 5,138,388(Kamp et al); 07/673,509 (DeBoer et al); 07/621,691, now abandoned (DeBoer et al); and PCT Application No. 91/08815, filed Nov. 26, 1991 disclose a process in which a uniform layer of toner is applied to a surface. The toner is imagewise tacked, preferably with a laser, to the surface and then the untacked toner is cleaned off the surface, leaving a toner image corresponding to the tacked toner. The tacked toner image can then be transferred to another surface, or it can be fixed to the surface to which it is tacked. This process can provide extremely high resolution and high density with fine toner particles and a precise laser. It does not require the use of light sensitive materials.
In this process, tacking can be accomplished by softening a heat softenable layer on the image surface, softening a toner particle where it touches the image surface, or both. Toner can be applied by a device comparable to that conventionally used to develop electrostatic images, for example, a magnetic brush development station. However, cleaning is somewhat more difficult, since the loose toner must be cleaned off without disturbing what may be quite lightly tacked imagewise toner. A preferable approach to this cleaning problem is to use a magnetic brush cleaner employing hard magnetic carrier particles and a rotating magnetic core which provides a soft cleaning brush that will clean off the loose toner while leaving the lightly tacked toner image.
A problem with the basic system is the large amount of toner that is cleaned off if a line or text image is made. It, thus, becomes desirable to recycle the toner from the cleaning station to the toner applying station. It is known generally in electrostatic imaging to recycle toner by conveying toner from a cleaning station to a toning station using augers or the like. A number of copiers and printers have been marketed with this feature.
The above applications disclose the use of a single magnetic brush to both apply toner and clean toner. Adjustments to the brush must be made using a substantially different bias for the two functions. This latter approach has the substantial advantage of automatically recycling the toner that is cleaned and eliminating the need for the auger or other conveying mechanism (as well as eliminating one station). However, it relies totally on bias to distinguish between applying and cleaning steps and, therefore, has somewhat less robustness than a two station system in which not only is bias altered, but the concentration of toner is altered between the stations.
U.S. Pat. No. 5,148,220, issued Sep. 15, 1992, Hilbert et al, shows a color electrophotographic image forming apparatus in which two color development stations are constructed as a single unit, which unit is spring urged toward a photoconductive drum. Four disk-shaped spacing elements are located coaxial with applicators in each station and provide a critical spacing for the applicators with respect to the drum. The applicators are maintained at their critical spacing whether applying toner or not. A magnetic valving structure is used to turn the stations on and off without changing their position with respect to the drum.